1. Field of the Invention
The present invention relates to a booster including a switching element, and more particularly, relates to a technique for reducing switching loss.
2. Description of the Background Art
Hybrid cars enhance fuel economy and exhaust emission characteristic by means of a combination of an engine and a motor as drive sources. For better fuel economy, for example, technology development has been underway to enhance drive efficiency of a motor. By way of example, a motor with a power supply voltage that is increased by a booster can be driven at a lower current than a motor providing the same output. A motor driven at a low current provides less resistance loss, leading to enhanced drive efficiency of a motor, an exemplary technique of which is introduced in example of such a technique is introduced in “TOYOTA Hybrid System THS II”, <http://www.toyota.co.jp/company/eco/ths2/kouden.html> (Accessed 6 Oct. 2003), a website of TOYOTA Motor Corporation.
Rise in a boosted voltage causes current reduction in a motor, leading to still less resistance loss which further enhances drive efficiency. On the other hand, a booster and an inverter each normally include a switching element. That is, rise in a boosted voltage results in increase of losses such as steady-state loss. In response, a boosted voltage is controlled to be at a level that allows optimization of drive efficiency and losses.
Incidentally, a surge voltage is developed when a switching element is actuated. Such a surge voltage is superimposed on a boosted voltage, and the resultant voltage is applied across the collector and the emitter of the switching element in the off state. In view of this, the sum of the surge and boosted voltages should not be more than a breakdown voltage of the switching element to avoid breakage of the switching element.
Other documents relevant to the present invention are as follows:
Japanese Patent Application Laid-Open No. 07-076280 (1995)
Japanese Patent Application Laid-Open No. 2002-112408
Japanese Patent Application Laid-Open No. 05-184180 (1993)
As discussed, a boosted voltage is controlled taking drive efficiency and losses of a motor into consideration, which means the allowable range of a surge voltage changes according to the level of the boosted voltage. Increase in switching speed of a switching element within the allowable range of a surge voltage in response to the level of the boosted voltage results in reduction in switching loss. However, the booster in the background art has a fixed switching speed relative to the maximum level of the boosted voltage. A hybrid car equipped with such a booster suffers heavy losses, thus resulting in poor fuel economy, for example.